Solar cells are generally made of semiconductor materials, such as silicon (Si), which convert sunlight into useful electrical energy. Solar cells are typically made of thin wafers of Si in which the required PN junction is formed by diffusing phosphorus (P) from a suitable phosphorus source into a P-type Si wafer. The side of silicon wafer on which sunlight is incident is in general coated with an anti-reflective coating (ARC) to prevent reflective loss of incoming sunlight, and thus to increase the efficiency of the solar cell. A two dimensional electrode grid pattern known as a front contact makes a connection to the N-side of silicon, and a coating of aluminum (Al) on the other side (back contact) makes connection to the P-side of the silicon. These contacts are the electrical outlets from the PN junction to the outside load.
Front and back contacts of silicon solar cells are typically formed by screen-printing a thick film paste. Typically, the front paste contains approximately fine silver particles, glass and organics. After screen-printing, the wafer and paste are fired in air, typically at furnace set temperatures of about 650-1000° C. During the firing, glass softens, melts, and reacts with the anti-reflective coating, etches the silicon surface, and facilitates the formation of intimate silicon-silver contact. Silver deposits on silicon as islands. The shape, size, and number of silicon-silver islands determine the efficiency of electron transfer from silicon to the outside circuit.
The back side of the silicon wafer typically includes Al paste, but generally lacks an ARC. Conventional back Al pastes do not fire through typical ARC materials such as SiNx, SiO2, and TiO2. Conversely, pastes that fire through well on the front side of silicon do not form a Back Surface Field (BSF) layer, and are hence unsuitable for use in solar cell back contacts.